This invention relates to the generation of pulsed sinusoidal signals wherein the nominal frequency of each pulsed signal is preselectable and, more particularly, to the use of dispersive and compressive filters to provide a time-expanded version of each of a set of possible output signals. Frequency selection and signal conditioning is accomplished at a relatively slow rate upon the expanded version of the signal prior to compression of each of the set of signals to a desired set of rapidly occurring output pulses.
The transmission of pulsed sinusoidal signals of differing frequencies is employed in communications, as well as in radar and sonar. Various terms are used to describe such form of communication, including frequency diversity, frequency shift keying, and frequency hopping. Various forms of circuits have been provided to generate sequences of pulsed sinusoids at differing frequencies.
For example, in the case of a transmission employing only two frequencies, a switching circuit may be employed to select either one of two frequencies. For transmission with many different frequencies, a frequency synthesizer may be employed, such synthesizer including oscillators, multipliers, counters, gating circuits, and other such circuits, which, as is well known, are employed for the generation of specific frequencies selected by suitable control signals from a keyboard or from a modem. Also, a phase-locked loop may be employed in which case counters or other form of division circuitry are employed within the loop to produce various output frequencies.
A problem arises in that the repetition rate of the pulses is limited to the rate at which the frequency can be changed from pulse to pulse. Such limitations appear from ringing within a circuit as a pulse is turned on and turned off, as well as from transients associated with a lack of phase coherence from pulse to pulse. Accordingly, existing communication systems are limited in that their data transmission rates from that which should be theoretically possible to transmit, if circuitry were available for providing the pulses to a transmission channel.